1. Field
The present invention relates to an optical element configured to diffuse lights from the light source.
2. Description of Related Art
Recently LED (light emitting diode) light sources have been widely used. Since a large portion of lights of a LED light source is emitted toward the front, an optical element configured to diffuse lights from the LED light source is commonly used in combination with the LED light source. Particularly, when LED light sources are used as light sources of an illumination unit for illuminating a large area, such as that for backlight, optical elements configured to diffuse lights from the LED light sources over a large angle are used such that a compact illumination unit can be realized with a small number of LED light sources (for example, Patent Document 1).
An LED light source for a large amount of light consists of a light emitting chip for emitting shorter-wavelength lights such as blue light and a fluorescent material which emits longer-wavelength fluorescences such as green, yellow or red. In many cases, in such an LED light source, the light emitting chip for emitting shorter-wavelength lights is arranged at the center while the fluorescent material which emits longer-wavelength fluorescences is arranged around the light emitting chip. In such an LED light source, the position of the portion emitting shorter-wavelength lights and the position of the portion emitting longer-wavelength lights are dissimilar from each other. Accordingly, when the optical device is used to diffuse lights from the light source, in some cases there exist directions in which shorter-wavelength lights are stronger and directions in which longer-wavelength lights are stronger. As a result, in some cases the color of light may become bluish in some directions while may become reddish in other directions. That is, the color of light may vary depending on the direction. For the use in illumination units, it is not preferable that color of light varies depending on the direction. However, an optical element configured to diffuse lights from the light source, which can reduce color difference of lights which occurs due to direction, has not been developed so far.
Patent Document 1: JP2006-92983A (JP3875247B)
Accordingly, there is a need for an optical element configured to diffuse lights from the light source, which can reduce color difference of lights which occurs due to direction.